The present invention relates to a highly thermal-conductive silicon carbide composite material having excellent thermal conductive characteristics, having a light weight, and suitable as a heatsink for semiconductor parts such as a ceramic substrate or an IC package, a method for producing it and a heat dissipation device employing it.
Along with enlargement of the capacity of semiconductor elements and high-integration of semiconductor elements in the semiconductor field in recent years, it has been an important theme how to discharge heat energy generated from a semiconductor element to the exterior effectively. A semiconductor element is usually loaded on an insulating substrate such as a ceramic substrate. In such a case, generated heat from the semiconductor element is discharged to the exterior by means of a heatsink provided on e.g. the back side of the substrate, to secure performance characteristics of the semiconductor element.
Conventionally, copper (Cu) has been mainly used for the material of the heatsink. Although copper has a coefficient of thermal conductivity of as high as 390 W/mK at a temperature in the vicinity of room temperature, it has a high coefficient of thermal expansion of 17xc3x9710xe2x88x926/K, and accordingly, cracks or fractures may form on a ceramic substrate due to a difference in thermal expansion between the ceramic substrate (coefficient of thermal expansion: 7-8xc3x9710xe2x88x926/K) and the heatsink, by addition of thermal cyclings. Conventionally, when the ceramic substrate for a heat dissipation device is used in such a field that reliability is required, e.g. Mo/W having a small difference in the coefficient of thermal expansion with the ceramic substrate, has been used as the heatsink.
Although the above-described Mo/W heatsink has an excellent reliability, it has a coefficient of thermal conductivity of as low as 150 W/mK, such being problematic in view of heat dissipation characteristics, and further, such a heatsink is expensive. Under these circumstances, an attention has been drawn to a metal-ceramic composite which comprises ceramic fibers or particles and copper or aluminum alloy reinforced by the ceramic fibers or particles, which is referred to simply as MMC (Metal Matrix Composite) in recent years. Such a composite is usually made in such a manner that ceramic fibers or particles as a reinforcing material are preliminarily formed to prepare a preform, and a metal as a base material (matrix) is infiltrated into the fibers or particles of the preform. As the reinforcing material, a ceramic such as alumina, silicon carbide, aluminum nitride, silicon nitride, silica or carbon may be employed. However, wettability of the ceramic as the reinforcing material and the alloy as the matrix, and the reaction layer at the interface therebetween, significantly affect the coefficient of thermal conductivity of the composite.
For the above-mentioned composite, to increase the coefficient of thermal conductivity, it is necessary to select a reinforcing material and an alloy having a high coefficient of thermal conductivity, and to decrease the coefficient of thermal expansion, it is necessary to select a reinforcing material having a low coefficient of thermal expansion. Accordingly, a composite of silicon carbide with aluminum alloy has been mainly studied.
However, with respect to the heat dissipation device comprising a conventional ceramic substrate and heatsink bonded to each other, as mentioned above, when a heavy-metal material such as Mo or W is employed for the heatsink, the heat dissipation device will be heavy, and heat dissipation properties will not be adequate. On the other hand, when e.g. Cu or Al, being relatively light and having excellent heat dissipation properties, is used as the heatsink, the difference in thermal expansion with the ceramic substrate will be large, and in order to obtain a structure with high reliability, the bonding structure itself will be extremely complicated, thus leading to increase in production costs and increase in thermal resistance as the heat dissipation device. Accordingly, with respect to the conventional heat dissipation device having bonding structure of the ceramic substrate and the heatsink, it has been objects to simplify the bonding structure, and to improve reliability and heat dissipation properties.
On the other hand, to overcome the above-mentioned problems, a metal-ceramic composite has been studied. However, in order to obtain a coefficient of thermal expansion close to the ceramic substrate, it is necessary to increase the ratio of the ceramic as a reinforcing material having a low coefficient of thermal expansion. To increase the ratio of the ceramic component, it is necessary to form a preform under a high forming pressure, whereby the cost will be high, and subsequent infiltration of the alloy may not be adequately carried out. Accordingly, it has been an object to develop techniques to provide a metal-ceramic composite having a coefficient of thermal expansion close to the ceramic substrate and having a high coefficient of thermal conductivity with a low cost.
Further, when such a composite is used as the heat dissipation device, the composite will be soldered to a circuit substrate, and accordingly, if the warpage of the composite is too large, the soldering will be difficult. Accordingly, when such a composite is used as the heat dissipation device, it is required to control the warpage within a certain amount. On the other hand, a device having such a heat dissipation device incorporated therewith, such as a power module, is usually fixed on e.g. a heat dissipation fin by screws. In such a case, the bonding surface between the device such as a power module and the heat dissipation fin is preferably convex so that a stress is applied on the bonding surface, in view of heat dissipation property since screw force after screwing will be high. However, with respect to the conventional metal-ceramic composite, in order to optionally add a shape such as warpage, as mentioned above, there is no way except adjustment by mechanical processing. In such a case, the metal-ceramic composite will be extremely hard, the mechanical processing cost will be high, and the device itself will be extremely expensive.
Under these circumstances, the present invention has been made to provide a composite which has a high thermal conductivity, a small specific gravity and a coefficient of thermal expansion close to the ceramic substrate, which has warpage, and which can be tightly bonded to e.g. a heat dissipation device, and a heat dissipation device employing it, with a low cost.
The present inventors have been made extensive studies to achieve the above-mentioned objects, and as a result, they have found that characteristics such as the coefficient of thermal expansion and the shape of the composite can be controlled by adjusting the composition and the structure of the composite, and the present invention has been accomplished.
Namely, the present invention provides a silicon carbide composite which is a flat composite comprising a porous preform of silicon carbide and a metal containing aluminum as the main component, infiltrated into the porous preform, said composite having a warpage of at most 250 xcexcm per 10 cm of the principal plane length of the composite.
The present invention further provides a silicon carbide composite which is a flat composite having at least 4 holes in its plane, and having a relation of 50xe2x89xa6Cxxe2x89xa6250 and xe2x88x9250xe2x89xa6Cyxe2x89xa6200, where Cx is a warpage (xcexcm) per 10 cm in a hole-to-hole direction (X direction) and Cy is a warpage (xcexcm) per 10 cm in a direction perpendicular thereto (Y direction).
The present invention further provides a silicon carbide composite having both front and back sides covered with a metal layer containing aluminum as the main component with an average thickness of from 10 to 150 xcexcm, with a difference in the average thickness between the front and back metal layers of at most 140 xcexcm.
The present invention further provides a silicon carbide composite which is a flat composite comprising a composite portion (A) and a metal layer (B) containing aluminum as the main component, provided on at least one side of the composite, and having a ratio of TA/TB of from 5 to 30, where TA is the average thickness (xcexcm) of the composite portion (A) and TB is the total of the average thicknesses (xcexcm) of the metal layers (B) on both sides.
The present invention further provides the above-mentioned silicon carbide composite, wherein the warpage is from 50 to 250 xcexcm per 10 cm of the principal plane length of the composite, and the product of |TB1xe2x88x92TB2| and the maximum length of the composite (L; cm) is from 500 to 2500, where |TB1xe2x88x92TB2| is the absolute value of the difference between the average thickness of the front metal layer (B) (TB1; xcexcm) and the average thickness of the back metal layer (B) (TB2; xcexcm).
The present invention further provides a silicon carbide composite comprising a porous preform of silicon carbide and having a stepped portion on at least one principal plane of the preform.
The present invention further provides a silicon carbide composite which is a composite having two flat composites (C and D) and metal layers (E) containing aluminum as the main component, laminated to have a structure of ECEDE, wherein the difference in the carbon content between the flat composites (C) and (D) is from 0.5 to 2.5 wt %, and the warpage is from 50 to 250 xcexcm per 10 cm of the principal plane length of the composite.
The present invention further provides a method for producing a silicon carbide composite, which comprises applying stress on a silicon carbide composite at a temperature of at least 350xc2x0 C. for plastic deformation, to obtain warpage.
The present invention further provides a silicon carbide composite which has an average coefficient of thermal expansion of at most 9xc3x9710xe2x88x926/K when heated from room temperature (25xc2x0 C.) to 150xc2x0 C., and a coefficient of thermal conductivity of at least 150 W/mK at room temperature (25xc2x0 C.).
The present invention further provides a heat dissipation device which comprises a flat composite and a ceramic substrate for semiconductor bonded thereto.
The present invention further provides a heat dissipation device which comprises a ceramic substrate made of aluminum nitride and/or silicon nitride.
The present invention further provides the above-mentioned heat dissipation device, wherein when the side having no ceramic substrate bonded thereto is subjected to bonding to a plate by means of a heat dissipation grease, at least 90% of said side adheres to the plate under a screw torque of at least 2N.